Do you feel pain and discomfort in your knees for the last several days, and it is not improving even after rest? If your answer is yes, you need to see your healthcare practitioner and get checked for swollen knees.

Swollen knees are also known as "knee effusion." Most commonly, it is called "water on the knees." This condition occurs when fluid builds up in or around the knee joints.

A brief overview of knee structure

The knee, the largest joint in your body, fulfils a vital purpose.¹ It is a complex combination of bones, tendons, and muscles that allows you to participate in various activities such as walking, running, and jumping while ensuring an active and healthy lifestyle. 

The knee joint consists of three bones, all interconnected to each other

• The thigh bone (Femur)
• The kneecap (Patella)
• The shin bone (Tibia)

Swelling indicates that the knee is inflamed. Fluid accumulation around the knee joint results in enlargement and puffiness of the injured area.

Various types of inflammation can occur in joints like the knee. These may include:

• Effusion: the swelling within the joint.
• Edema: swelling in the tissues surrounding the joint.
• Hemarthrosis: manifested as swelling and the accumulation of blood within the joint.

Symptoms of swollen knees

Common signs and symptoms often consist of:

Swelling: There is noticeable puffiness around the kneecap, particularly when compared to the unaffected knee.

Stiffness: It becomes challenging to fully bend or straighten the leg due to excess fluid in the knee joint.

Pain: The intensity of the pain can vary depending on the root cause of the fluid buildup, potentially making it too unbearable to put weight on the knee.

Causes of swollen knees

The causes of swollen knees may be classified into two categories.

• Traumatic 
• Nontraumatic

Traumatic causes

Traumatic causes indicate that knee swelling is usually due to injury. The most common traumatic cause is overuse, such as in sports. 

A severe injury, such as a knee strike, accident, hard fall, or dislocation, may harm the knee and cause effusion.

Some examples of traumatic knee injuries are:

Fracture

The patella frequently suffers fractures, making it the most commonly broken bone around the knee. Fractures can also occur at the spot where the femur and tibia connect to make the knee joint.²

Dislocation

A dislocation is a condition in which the bones of the knee get out of position either entirely or partially, causing discomfort and pain in the knee.²

Anterior Cruciate Ligament (A.C.L.) Injuries

The anterior cruciate ligament (A.C.L.) is an important ligament that aids knee joint stability. The A.C.L. joins the thigh bone (femur) to the shinbone (tibia). It is most commonly damaged during sports that demand sudden stops and changes of direction, such as basketball, soccer, tennis, and volleyball.²

Posterior cruciate ligament injuries

The posterior cruciate ligament is usually damaged when bent. A hit to the front of the knee injures it. Posterior cruciate ligament injuries are typically partial and self-healing.²

Torn knee cartilage

Torn knee cartilage, such as meniscus tears, usually occurs during sports. Injuries in the meniscus may result from twisting, cutting movements, pivoting actions, or being tackled. Meniscal tears are also possible due to arthritis or ageing.²

Tendon tears

The quadriceps and patellar tendons can be strained or injured. Middle-aged individuals who participate in activities like running or jumping are at a higher risk of experiencing tendon tears, although injuries can happen to anyone.

Non-traumatic causes

Non-traumatic causes are the causes which are not due to injury or trauma. These injuries arise due to

• Infections
• Inflammatory conditions
• Tumours

Infections

Knee infection, commonly bacterial, can arise when a germ enters the body through a skin breach (for example, following an open fracture, knee surgery, or knee injection). It may also be due to a whole-body infection. It infects the joint by spreading germs through the bloodstream.

The most common examples are 

• Cellulitis occurs when a local bacterial skin infection spreads and invades nearby tissues
• Septic arthritis is caused by a systemic bacterial, viral, or fungal infection penetrating synovial fluid and joint structures

Inflammatory conditions

Swollen knees may also be the result of inflammatory conditions, which may induce persistent or severe inflammation.

Several knee inflammatory conditions are autoimmune, where the immune system wrongly triggers inflammation in normal tissues. Other disorders induce knee inflammation when the knee joint deteriorates, forcing bone to scrape against bone.

Examples of inflammatory conditions include:

Rheumatoid arthritis

A chronic disease that causes inflammation around the body and commonly presents with pain in the joints.³

Osteoarthritis

Osteoarthritis is a degenerative joint disease in which the tissues in the joint break down over time.⁴ 

Psoriatic arthritis

Psoriatic arthritis can occur in persons with psoriasis, frequently causing joint discomfort and swelling.⁵

Gout

Gout develops when urate crystals collect in your joint, producing inflammation and extreme pain during a gout episode. When uric acid levels are high in the body, crystals of urate form.⁶

Pseudogout

Pseudogout, also called false gout, is a form of arthritis that mimics gout but has calcium pyrophosphate crystal deposition.

Tumours

Specific tumours may attack the nearby knee joint and induce outflow, resulting in swelling. Some of them are as follows.,

Enchondroma

Enchondroma is a non-cancerous tumour inside the cartilage of your bones. This tumour rarely shows signs of pain or other symptoms.

Chondroblastoma

Chondroblastoma is a rare, non-cancerous bone tumour that tends to grow in the ends of long bones, like where the joints are. It starts from young cartilage cells called chondroblasts that grow too much in some regions of the bone. These areas are usually where new bone is forming during growth.

Chondroblastoma grows in joints at the ends of long bones. It is a rare, non-cancerous form of bone tumour. This tumour originates from immature outgrowth of young cartilage cells called chondroblasts. The femur is usually the most commonly affected site, followed by the humerus and tibia.⁷

Management of swollen knee

The first step in the management is to find out the underlying cause. 

Your healthcare provider may perform a thorough physical examination of the swollen knee and take a complete history to establish the cause of the pain and swelling. 

Based on the physical examination and history, the healthcare provider may request the following tests to confirm the diagnosis further.

Imaging test

Imaging tests may help evaluate the root cause of the swollen knee.

These include:

X-ray

The X-ray is done to evaluate whether the swelling in the knee is due to dislocated joints or arthritis. 

Ultrasound

Ultrasound using sound waves determines whether the pain and swelling are due to disorders affecting ligaments and tendons.

M.R.I

MRI scans use radio waves to identify injuries in tendons, ligaments, and other soft tissues that may not be visible through X-ray imaging.

Joint aspirations

This procedure is called arthrocentesis. In this procedure, a sample of synovial fluid is taken from the joint space and analysed. The results may provide insights into potential infection, inflammation, blood, or uric acid crystals.

Treatment of swollen knee

Treatment of swollen knees starts with the R.I.C.E. method. The R.I.C.E. method includes rest, ice, compression and elevation.

Rest: Rest includes rest! It means trying to avoid overuse of the knees. Taking a break from the activities that cause you knee pain and discomfort.⁸

Ice: Applying an ice or cold pack is essential to prevent or minimise swelling. Cold treatment mainly relieves pain and swelling. Apply the ice or cold pack thrice daily for 10 to 20 minutes. 

You may switch to applying heat to the affected area after 48 to 72 hours when swelling has subsided. 

Compression: Apply a compression bandage to minimise swelling. However, avoid covering it too tightly since this might cause more oedema. 

Elevation: As often as possible, elevate your knees and legs over your heart. It helps reduce pain and swelling.

If the damage is severe, you may require a knee brace and a mobility aid, such as crutches.

Physical therapy may be recommended to restore strength and range of motion after an accident, fracture, or prolonged immobilisation. 

Medications

Several medications are prescribed for swollen knees, depending upon the cause.

Some of these are 

• NSAIDs
• Antibiotics
• Corticosteroids
• D.M.A.R.D.S (for autoimmune diseases)

Surgery

Surgery is only recommended when other treatments fail to relieve the symptoms.

If the swelling is caused by an injury, such as a torn ligament or meniscus tear, you may require a knee arthroscopy to repair the damage to your knee.

Suppose your arthritis and symptoms such as swelling and discomfort in your knee make it challenging to carry out your normal activities. In that case, your doctor may propose a knee replacement called arthroplasty. 

Summary

• Swollen knees, or knee effusion, is due to the fluid build-up in or around the knee joint
• Causes of swollen knees may be traumatic such as sports injury, fractures, dislocation, tendon or ligament tear
• Non-traumatic causes include infections like cellulitis and septic arthritis
• Other causes are inflammatory conditions like arthritis, osteoarthritis and gout
• Several tumours may also be the reason for swollen knees
• Diagnosis involves a physical exam, history, X-rays, MRIs, and joint aspiration tests
• The R.I.C.E. method accounts for the initial treatment which stands for rest, ice, compression, and elevation
• Medications like NSAIDs, antibiotics, and corticosteroids may be prescribed depending on the cause
• Severe cases might require physical therapy or surgery, such as knee arthroscopy or replacement for arthritis